Japanese Patent No. 4128194 discloses a counter-rotating axial flow fan which includes a casing including an air channel having a suction port on one side in an axial direction and a discharge port on the other side in the axial direction, a front impeller including a plurality of front blades and configured to rotate in the air channel, a rear impeller including a plurality of rear blades and configured to rotate in the air channel, and a middle stationary portion formed by a plurality of stationary blades or struts disposed to be stationary between the front impeller and the rear impeller in the air channel.
In such conventional counter-rotating axial flow fan, the front impeller, the rear impeller, and the middle stationary portion are elaborately shaped to reduce noise. It has been found that noise at the target operating point can be reduced by optimizing the design of the front impeller, the rear impeller, and the middle stationary portion. In practice, however, a counter-rotating axial flow fan may be operated at an operating point (desired target operating point) more or less deviated from the target operating point that has been set in the initial design, which consequently leads to increased noise.